You have a condensation issue. Your building is either not insulated good enough, has a venting issue and possibly a vent fan expelling water into the attic.
To get a really good answer to your situation, get a Home inspector out there for an evaluation. A good contractor or really good roofer may also be able to give you sage advice. Try American Society of Home Inspectors, ASHI
for a reputable inspector.
You need to get your fan and bath vents to the exterior, and you need to insulate for r-50 in your area with no penetrations.
Good luck, let us know what happens.